0 Automatic AC Power Switch

for the Holiday Home Electrical appliances accidentally left on  in (holiday) homes left unoccupied for a  short or a long period consume power  unnecessarily and can present a fire hazard. Everyone will be familiar with those  nagging thoughts, a few miles down the  road from the house: “Did I remember  to switch off the coffee machine? The  lights? The oven?” 

Automatic AC Power Switch Circuit diagram :

Automatic AC Power Switch Circuit Diagram

Hotel rooms are often equipped with a  switch near the main door which enables the power supply to everything in  the room only when the plastic card (which  might contain a chip or have a magnetic strip  or a pattern of holes) that serves as the room  key is inserted. The circuit idea given here  to switch off lights and other appliances is  along the same lines. The solution is surprisingly simple. 

A reed contact is fitted to the frame of the main entrance door, and a matching magnet  is attached to the door itself such that when  the door is closed the reed contact is also  closed. To enable power to the house, press  S1 briefly. Relay RE1 will pull in and complete  the circuit for all the AC powered appliances in  the house. The relay will be held in even after  the button is released via the second relay contact and the reed contact (‘latching’ function). 

As soon as the main entrance door is  opened, the reed contact will also open.  This in turn releases the latch circuit and  consequently the relay drops out. The  various connected appliances will thus  automatically and inevitably be switched  off as soon as the house is left. The circuit is principally designed for  small holiday homes, where this mode  of operation is particularly practical. Of course, for any circuit that deals in AC  powerline voltages, we must mention  the following caution. 

Caution:

shock hazard! Construction and connection of this circuit  should only be carried out by suitably-qualified  personnel, and all applicable electrical safety  regulations must be observed. In particular, it  is essential to ensure that the relay chosen is  appropriate for use at domestic AC grid volt-ages and is suitably rated to carry the required  current.

Detail: Automatic AC Power Switch

0 Protection For Your Electrical Appliances

Here is a very low-cost circuit to save your electrically operated appliances, such as tv, tape recorder, refrigerator, and other instruments during sudden tripping and resumption of mains supply. Appliances like refrigerators and air-conditioners are more prone to damage due to such conditions. The simple circuit given here switches off the mains supply to the load as soon as the power trips. The supply can be resumed only by manual intervention. Thus, the supply may be switched on only after it has stabilised.

Protection For Your Electrical Appliances Circuit diagram :

Protection For Your Electrical Appliances

The circuit comprises a step-down transformer followed by a full-wave rectifier and smoothing capacitor C1 which acts as a supply source for relay rl1. Initially, when the circuit is switched on, the power supply path to the step-down transformer X1 as well as the load is incomplete, as the relay is in de-energised state. To energise the relay, press switch S1 for a short duration. This completes the path for the supply to transformer X1 as also the load via closed contacts of switch S1. Meanwhile, the supply to relay becomes available and it gets energised to provide a parallel path for the supply to the transformer as well as the load.

If there is any interruption in the power supply, the supply to the transformer is not available and the relay de-energises. Thus, once the supply is interrupted even for a brief period, the relay is de-energised and you have to press switch S1 momentarily (when the supply resumes) to make it available to the load. Very short duration (say, 1 to 5 milliseconds) interruptions or fluctuations will not affect the circuit because of presence of large value capacitor which has to discharge via the relay coil. Thus the circuit provides suitable safety against erratic power supply conditions.

Author : MALAY BANERJEE

Detail: Protection For Your Electrical Appliances

0 On-Demand WC Fan Using 555

In most WCs with an extractor the fan is connected to the lighting circuit and is switched on and off either in sympathy with the light or with a short delay. Since toilets are sometimes used for washing the hands or just for a quick look in the mirror, it is not always necessary to change the air in the smallest room in the house. The following circuit automatically determines whether there really is any need to run the fan and reacts appropriately. No odour sensor is needed: we just employ a small contact that detects when and for how long the toilet seat lid is lifted.

On-Demand WC Fan circuit Using 555

If the seat lid is left up for at least some presettable minimum time t1, the fan is set running for another presettable time t2. In the example shown the contact is made using a small magnet on the lid and a reed switch mounted on the cistern. The rest is straightforward: IC2, the familiar 555, forms a timer whose period can be adjusted up to approximately 10 to 12 minutes using P2. This determines the fan running time. There are three CMOS NAND gates (type 4093) between the reed switch and the timer input which generate the required trigger signal. When the lid is in the ‘up’ position the reed switch is closed.

Capacitor C1 charges through P1 until it reaches the point where the output of IC1a switches from logic 1 to logic 0. The output of IC1b then goes to logic 1. The edge of the 0-1 transition, passed through the RC network formed by C2 and R2, results in the output of IC1c going to logic 0 for a second. This is taken to the trigger input on pin 2 of timer IC2, which in turn switches on the relay which causes the fan to run for the period of time determined by P2. The circuit is powered from a small transformer with a secondary winding delivering between approximately 8 V and 10 V. Do not forget to include a suitable fuse on the primary side.

The circuit around IC1b and IC1c ensures that the fan does not run continuously if the toilet seat lid is left up for an extended period. The time constant of P1 and C1 is set so that the fan does not run as a result of lavatorial transactions of a more minor nature, where the lid is opened and then closed shortly afterwards, before C1 has a chance to charge sufficiently to trigger the circuit.

Detail: On-Demand WC Fan Using 555

0 Outdoor LED Solar Lights Circuit Schematic

This Outdoor LED Solar Garden Lights project is a hobby circuit of an automatic garden light using a LDR and 6V/5W solar panel. During day time, the internal rechargeable 6 Volt SLA battery receives charging current from the connected solar panel through polarity protection diode D9 and current limiting resistor R10. If ambient light is normal, transistor T1 is reverse biased by IC1 (LM555). Here IC1 is wired as a medium current inverting line driver, switched by an encapsulated light detector (10mm LDR). Multi-turn trimpot P1 sets the detection sensitivity. When ambient light dims, transistor T1 turns on to drive the white LED string (D1-D8). Now this lamp load at the output of T1 energizes. Resistors R1-R8 limits the operating current of the LEDs. When the ambient light level restores, circuit returns to its idle state and light(s) switched off by the circuit.

Outdoor LED Solar Lights Circuit Schematic


 

Outdoor Garden Solar Lights Circuit Diagram

Assemble the Outdoor Solar Lights circuit on a general purpose PCB and enclose the whole assembly in a transparent plastic box. Drill suitable holes on the top of the enclosure to mount the mini solar panel (SP1) and the light sensor (LDR), and in front for fitting power switch (S1) and the sensitivity controller (P1). Fix the battery inside the cabinet using a double-sided glue tape/pad. Finally, the LDR should not be mounted to receive direct sunlight. It must be mounted at the top of the enclosure, pointing to the sky say southwards. This circuit is very simple. So interested and experienced hobbyists can alter/modify the whole circuit as per their own ideas without any difficulty (Just try a 6V relay with T1 to drive more number of LED strings).

Detail: Outdoor LED Solar Lights Circuit Schematic

0 12 V AC Dimmer

The circuit described here is derived from a conventional design for a simple lamp dimmer, as you can see if you imagine a diac connected between points A and B. The difference between this circuit and a normal diac circuit is that a diac circuit won’t work at 12 V. This is the fault of the diac. Most diacs have a trigger voltage in the range of 30 to 40V, so they can’t work at 12 V, which means the dimmer also can’t work.

12 V AC Dimmer Circuit diagram :

12-V AC Dimmer Circuit Diagram

The portion of the circuit between points A and B acts like a diac with a trigger voltage of approximately 5.5 V. The network formed by R1, P1 and C1 generates a phase shift relative to the supply voltage. The ‘diac equivalent’ circuit outputs a phase-shifted trigger pulse to the triac on each positive and negative half-cycle of the sinusoidal AC voltage.

This works as follows. First consider the positive half of the sine wave. C1 charges when the voltage starts to rise, with a time constant determined by C1, R1 and P1. T1 does not start conducting right away. It waits until the voltage across D2 reaches 4.7 V and the Zener diode starts to conduct. Then current starts to flow, driving T1 and T3 into conduction. This produces a pulse at point B. The same principle applies to the negative half of the sine wave, in this case with D1, T2 and T4 as the key players.

The trigger angle can be adjusted with P1 over a range of approximately 15 degrees to 90 degrees. C2 provides a certain amount of noise decoupling. Depending on the load, the triac may need a heat sink. You can use practically any desired transistors; the types indicated here are only examples. If the circuit does not dim far enough, you can change the value of P1 to 25 kΩ. This allows the trigger angle to be increased to 135 degrees.

Note: this circuit works fine with normal transformers, but not with ‘electronic ’ transformers.

Detail: 12 V AC Dimmer

0 Automatic Curtain Opener

This circuit can be used with a timer clock to open and close curtains or (vertical) Venetian blinds. The curtain or blind is driven by  an electric motor with a reduction gearbox fitted to the control mechanism of the curtain or blind. This circuit is ideal for giving your home an occupied appearance while you are away on holiday or for some other reason. In the author’s house, this arrangement has provided several years of trouble-free service on a number of windows fitted  with Venetian blinds. 

The original design was a simple relay circuit with pushbuttons for opening and closing and reed switches acting as limit switches. The mechanical drive is provided by a small DC motor with a reduction gearbox and pulley (all from Conrad Electronics).  It was later modified to work automatically with a timer clock. The timer operates a small  230-VAC (or 120-VAC) relay with a changeover contact. Thanks to the two timers, the motor stops after a few seconds if one of the reed switches is missed due to a mechanical defect. 

Automatic Curtain Opener Circuit diagram :

Automatic Curtain Opener Circuit Diagram

 

The circuit works as follows (see Figure 1). In the quiescent state, relays RE1–RE3 are de-energised and the motor is stopped. Open the blind: 

When the timer clock applies power to the 230-V (120-V) relay RE3, the voltage at the junction of C1 and R1 goes high. IC1 (a 555)  then receives a trigger pulse on pin 2, which causes its output (pin 3) to go High and energise RE1, which in turn causes the motor to start running. When the magnet reaches reed  switch S1 (‘Open’), the 555 is reset. If the reed  switch does not operate for some reason, the relay is de-energised anyhow when the  monostable times out (time delay = 1.1 RC;  approximately 5 seconds). Close the blind:

The timer clock removes power from RE3, which causes a trigger pulse to be applied to the other 555 timer (IC2) via R5 and C4. Now the motor starts running in the other direction. The rest of the operation is the same as described above for opening the blind. Diodes D2 and D5 prevent the outputs of the 555 ICs from being pulled negative when the relay is de-energised, which could otherwise cause the timer ICs to malfunction. 

All  components  of  the  mechanical  drive  come from Conrad Electronics [2]: a motor with a reduction gearbox (type RB32, order number 221936) and a pulley (V-belt pulley, order number 238341) on the output shaft. An O-ring is fitted to the pulley to provide  sufficient friction with the drive chain of the Venetian blind. The magnet for actuating the  reed switches is a rod magnet with a hole in the middle (order number 503659), and the chain of the Venetian blind is fed through this hole.

Author : Ton Smits  - Copyright : Elekto

Detail: Automatic Curtain Opener

0 Garage Door Closing Circuit Just using Relays

Because I’m old school, I wanted to build a Garage Door Closing circuit without relying on integrated configurations (555 timer etc) to keep it simplistic. The circuit closes the garage door after two minutes with C3 and four minutes with the addition of C2. The timer relay is surprisingly accurate (+/- five seconds). Another feature is to ensure that the garage door actually did close, such as if it’s stopped mid-operation by the user.

Description:

S3 (magnetic N.C.) is located at the garage door and activates the circuit when the garage door opens.
RL1 is the reset timer. It’s maintained in the “on” position for two minutes by C3 while the trigger capacitor, C4, is charged. RL2 is the conduit, directing C4 to either RL3 or R1 to ground when off. Purpose of R1 is to prevent arching across contacts and a fast discharge. RL3’s contacts are connected to the Garage Door’s Momentary Switch and is sustained “on”  for a half second by C5.

When C3 discharges to the cutoff voltage of RL1, it turns off and resets. C4 charges C5, which turns on RL3 and initiates the garage door. Because C4 does not have the time to fully discharge, it should be at least three times the value of C5. If it does not close, RL1 in countdown mode will reset and open the door. When it resets again, the door will close.

Turning off the circuit, C1 maintains RL1 “on” slightly longer to ensure that RL2 is set to discharge C4 to R1. If this is not done and C4 is not discharged, the garage door will not open until it discharges naturally and falls below the trigger voltage for RL3.  The circuit would be useless for several days.

Notes:

• Time delay of RL1 after reset drops 15 seconds because of the short charge time.
• To boost RL3 to a one-second delay, increase C5 to 1000uF.
• D2, D3, and D4 isolate the crucial sections of the circuit.
• Relays do not turn off at the same rate. I conducted a test by tripping the circuit on and off at a high rate and discovered the possibility of C4 turning on RL3. The addition of C1 solved this.

Author: Roland Segers (speedmail-at-gmail.com)

Detail: Garage Door Closing Circuit Just using Relays

0 Remote-Controlled Fan Regulator Circuit Diagram

Using this circuit, you can change the speed of the fan from your couch or bed. Infrared receiver module TSOP1738 is used to receive the infrared signal transmitted by remote control. The circuit is powered by regulated 9V. The AC mains is stepped down by transformer X1 to deliver a secondary output of 12V-0-12V. The transformer output is rectified by full-wave rectifier comprising diodes D1 and D2, filtered by capacitor C9 and regulated by 7809 regulator to provide 9V regulated output. Any button on the remote can be used for controlling the speed of the fan. Pulses from the IR receiver module are applied as a trigger signal to timer NE555 (IC1) via LED1 and resistor R4.

Remote-Controlled Fan Regulator Circuit Diagram

Remote-Controlled Fan Regulator Circuit Diagram

IC1 is wired as a monostable multivibrator to delay the clock given to decade counter-cum-driver IC CD4017 (IC2).Out of the ten outputs of decade counter IC2 (Q0 through Q9), only five (Q0 through Q4) are used to control the fan. Q5 output is not used, while Q6 output is used to reset the counter. Another NE555 timer (IC3) is also wired as a monostable multivibrator. Combination of one of the resistors R5 through R9 and capacitor C5 controls the pulse width.  The output from IC CD4017 (IC2) is applied to resistors R5 through R9. If Q0 is high capacitor C5 is charged through resistor R5, if Q1 is high capacitor C5 is charged through resistor R6, and so on.

Optocoupler MCT2E (IC5) is wired as a zero-crossing detector that supplies trigger pulses to monostable multivibrator IC3 during zero crossing. Opto-isolator MOC3021 (IC4) drives triac BT136. Resistor R13 (47-ohm) and capacitor C7 (0.01µF) combination is used as snubber network for triac1 (BT136). As the width of the pulse decreases, firing angle of the triac increases and speed of the fan also increases. Thus the speed of the fan increases when we press any button on the remote control. Assemble the circuit on a general-purpose PCB and house it in a small case such that the infrared sensor can easily receive the signal from the remote transmitter.

Detail: Remote-Controlled Fan Regulator Circuit Diagram

0 Remote Washing Machine Alert

It is often the case these days that the washing machine and  tumble dryer are installed in an outbuilding  or corner of a garage. This not only makes the kitchen a much quieter place but also leaves room for a dish washer and gives additional cupboard space. The problem now is how to tell when the wash cycle is finished. In bad weather you don’t want to make too many fruitless trips down the garden path just to check if the wash cycle is finished. The author was faced with this problem when he remembered a spare wireless door chime he had. With a few additional components and a phototransistor to passively detect when the washing machine’s ‘end’ LED comes on, the problem was solved. 

Remote Washing Machine Alert Circuit diagram :

Remote Washing Machine Alert Circuit Diagram

C1 smoothes out any fluctuations in the LED light output (they are often driven by a multiplex signal) producing a more stable DC voltage to inputs 2 and 6 of IC1. The circuit is battery powered so the CMOS version of the familiar 555 timer is used for IC1 and IC2. The output of IC1 (pin 3) keeps IC2 reset (pin 4) held Low while there is no light falling on T1. When the wash cycle is finished the LED lights, causing T1 to conduct and the voltage on C1 starts to fall. Changing  the value of R1 will increase sensitivity if the LED is not bright enough. 

When the voltage on C1 falls  below 1/3 of the supply volt-age IC1 switches its output  (pin 3) High, removing the  reset from IC2. T2 conducts  and LED D1 is now lit, sup-plying current to charge C2.  When  the  voltage  across  C2 reaches 2/3 supply IC2  switches its output Low and  C2 is now discharged by pin  7 via R3. The discharge time  is roughly one minute before  the transistor is again switched on. The process repeats as long as light is falling on T1. 

Transistor T2 is a general-purpose small signal NPN type. The open collector output is  wired directly in parallel with the bell push  (which still functions if the transistor is not  switched on). Ensure that transistor output is  wired to the correct bell push terminal (not the side connected to the negative battery  terminal).

Each timer consumes about 60 µA quiescent and the circuit can be powered from the transmitter battery. Alternatively a 9 V battery can be substituted; it has much greater capacity than the original mini 12 V battery fitted in the bell push. Before you start construction, check the range of the wireless doorbell to make sure  the signal reaches from the washing machine to wherever the bell will be fitted. 

Author : Götz Ringmann - Copyright : Elektor

Detail: Remote Washing Machine Alert

0 An Electronic Watering Can

Summertime is holiday time but who will be looking after your delicate houseplants while you are away? Caring for plants is very often a hit or miss affair, sometimes you under-water and other times you over-water. This design seeks to remove the doubt from plant care and keep them optimally watered. 

The principle of the circuit is simple: first the soil dampness is measured by passing a signal through two electrodes placed in the soil. The moisture content is inversely proportional to the measured resistance. When this measurement indicates it is too dry, the plants are given a predefined dose of water. This last part is important for the correct function of the automatic watering can because it takes a little while for the soil to absorb the water dose and for its resistance to fall. If the water were allowed to flow until the soil resistance drops then the plant would soon be flooded. 

An Electronic Watering Can Circuit diagram :

An Electronic Watering Can Circuit Diagram

The circuit shows two 555 timer chips IC1 and IC2. IC1 is an astable multivibrator producing an ac coupled square wave at around 500 Hz for the measurement electrodes F and F1. An ac signal reduces electrode corrosion and also has less reaction with the growth-promoting chemistry of the plant. Current flowing between the electrodes produces a signal on resistor R13. The signal level is boosted and rectified by the voltage doubler produced by D2 and D3. When the voltage level on R13 is greater than round 1.5 V to 2.0 V transistor T2 will conduct and switch T3. Current flow through the soil is in the order of 10 µA. 

T2 and T3 remain conducting providing the soil is moist enough. The voltage level on pin 4 of IC2 will be zero and IC2 will be disabled. As the soil dries out the signal across R13 gets smaller until eventually T2 stops conducting and T3 is switched off. The voltage on pin 4 of IC2 rises to a ‘1’ and the chip is enabled. IC2 oscillates with an ‘on’ time of around 5 s and an ‘off’ time (adjustable via P2) of 10 to 20 s. This signal switches the water pump via T1. P1 allows adjustment of the minimum soil moisture content necessary before watering is triggered. 

The electrodes can be made from lengths of 1.5 mm2 solid copper wire with the insulation stripped off the last 1 cm. The electrodes should be pushed into the earth so that the tips are at roughly the same depth as the plant root ball. The distant between the electrodes is not critical; a few centimetres should be sufficient. The electrode tips can be tinned with solder to reduce any biological reaction with the copper surface. Stainless steel wire is a better alternative to copper, heat shrink sleeving can used to insulate the wire with the last 1 cm of the electrode left bare. Two additional electrodes (F1) are con nected in parallel to the soil probe electrodes (F). The F1 electrodes are for safety to ensure that the pump is turned off if for some reason water collects in the plant pot saucer. A second safety measure is a float switch fitted to the water reservoir tank. 

When the water level falls too low a floating magnet activates a reed switch and turns off the pump so that it is not damaged by running with a dry tank. Water to the plants can be routed through closed end plastic tubing (with an internal diameter of around 4 to 5 mm) to the plant pots. The number of 1 mm to 1.5 mm outlet holes in the pipe will control the dose of water supplied to each plant. The soil probes can only be inserted into one flowerpot so choose a plant with around average water consumption amongst your collection. Increasing or decreasing the number of holes in the water supply pipe will adjust water supply to the other plants depending on their needs. A 12 V water pump is a good choice for this application but if you use a mains driven pump it is essential to observe all the necessary safety precautions. 

Last but not least the electronic watering can is too good to be used just for holiday periods, it will ensure that your plants never suffer from the blight of over or under-watering again; provided of course you remember to keep the water reservoir topped up…

Author : Robert Edlinger

Detail: An Electronic Watering Can

0 Unique Water Pump Controller

Here is a simple solution for automatic pumping of water to the overhead tank. Unlike other water-level indicators,  it  does not use probes to detect the water level and hence there is no probe corrosion problem. It has no direct contact with water, so the chance of accidental leakage of electricity to the water tank is also eliminated. Two important advantages of the circuit are that the water level never goes below a particular level and no modification in the water tank is required. 

Unique Water Pump Controller Circuit diagram :

Fig.1 Unique Water Pump Controller Circuit diagram

Fig. 1 shows the circuit of the water-pump controller. The circuit uses an LDR-white LEDs assembly to sense the water level. It forms a triggering switch to energise the relay for controlling the pump. The LDR-LEDs assembly (shown in Fig. 2) is fixed on the inner side of the cap  of  the  water tank without making contact with water. The light reflected from  the water tank is used to control the resistance of LDR1.

Fig 2 Sensor circuit diagram

When the water level is high enough, light from the white LEDs (LED1 through LED3) reflects to fall on LDR1. This reduces the resistance of LDR1, increasing the voltage at the non-inverting input (pin 3)  of IC1. IC1  is used in the circuit as a  voltage comparator. Resistors R4 and R5 form a potential divider to fix half of supply voltage to the inverting input of IC1. 

Normally, when the water tank is full, LDR1 gets more of reflected light because the distance between the water level and the face of LDR1 is minimal. When white light falls on LDR1, the voltage at the non-inverting input (pin 3) of IC1 increases and its output goes high. This high output makes pnp transistor T1 non-conducting and the relay remains de-energised. LED1 also remains ‘off.’ Since the water-pump power supply is connected to the normally-open (N/O)  contacts of  relay RL1, pumping is stopped.

When water level falls, the amount of  light reflected to LDR1 decreases and its resistance increases. This reduces the  voltage at pin 3 of IC1 and its output goes  low. This  low output from IC1 makes transistor T1 conduct. Relay RL1 energises to close the N/O  contacts and the motor  starts pumping water. LED1 glows to indicate the pumping of water. 

Fig.3 Sensor assembly

 

Assemble the circuit on a general-purpose PCB and enclose in a suitable  cabinet. Solder the white LEDs-LDR1 assembly on a separate PCB and use a separate power supply for it. Mount LEDs behind the LDR. Otherwise, light from the LEDs will  affect the working of the circuit. Connect LDR1 to the main circuit board at ‘A’ and ‘B’ points. 

Fix the LEDs-LDR1 assembly on the inner side of the water-tank cap as shown in Fig.  3. Orient the LEDs and the LDR such that when the water tank is full, the light emitted from the LEDs and reflected  from the water surface falls directly on  LDR1.  The  distance between the upper level of water and the LEDs-LDR setup should be minimal, ensuring that water doesn’t touch  LDR1. Otherwise, the circuit  will  not function properly. By using more white  LEDs, this  distance  can  be increased. Cover the LDR with a black tube to increase its sensitivity. 

You can fix the main unit at a convenient place and connect it to the LEDs-LDR  assembly through wire. Select the relay according to the horse-power (HP) of the water pump. After  arranging the setup (with  maximum water in the tank), adjust VR1 until LED1 stops glowing. In this state, the relay should de-energise. When the water level decreases, the relay automatically energises to connect mains to the motor and it starts pumping water.

Author :D.Mohan Kumar - Copyright: EFY

Detail: Unique Water Pump Controller

0 Remote Control Blocker

This circuit was designed to block signals from infrared remote controls. This will prove very useful if your children have the tendency to switch channels all the time. It is also effective when your children aren’t permitted to watch TV as a punishment. Putting the TV on standby and put-ting the remote control out of action can be enough in this case. 

Remote Control Blocker Circuit diagram :

Remote Control Blocker Circuit Diagram

The way in which we do this is very straightforward. Two IR LEDs continuously transmit infrared light with a frequency that can be set between 32 and 41 kHz. Most remote controls work at a frequency of 36 kHz or 38 kHz. 

The disruption of the remote control occurs as follows. The ‘automatic gain’ of the IR receiver in TVs, CD players, home cinema systems, etc. reduces the gain of the receiver due to the strong signal from the IR LEDs. Any IR signals from a remote control are then too weak to be detected by the receiver. Hence the equipment no longer ‘sees’ the remote control! 

 

The oscillator is built around a standard NE555. This drives a buffer stage, which provides the current to the two LEDs. Setting up this circuit is very easy. Point the IR LEDs towards the device that needs its remote control blocked. Then pick up the remote control and try it out. If it still functions you should adjust the frequency of the circuit until the remote control stops working.

This circuit is obviously only effective against remote controls that use IR light!

Author : Paul Goossens - Copyright : Elektor

Detail: Remote Control Blocker

0 Fridge Thermostat

What to do when the thermostat in your fridge doesn’t work any more? Get it repaired at (too) much expense or just buy a new one? It is relatively simple to make an electronic variation of a thermo-stat yourself, while saving a considerable amount of money at the same time. How-ever, be careful when working with mains voltages. This voltage remains invisible and can sometimes be fatal! 

This design allows for five temperatures to be selected with a rotary switch. By selecting suitable values for the resistors (R1 to R7), the temperatures at the various switch positions can be defined at construction time. With the resistance values shown here, the temperature can be adjusted to 16, 6, 4, 2 and –22 °C. 16° C is an ideal temperature for the storage of wine, while 6, 4, and 2 degrees are interesting for beer connoisseurs and the minus 22°degrees position transforms the fridge into a large freezer. Note for wine connoisseurs: to prevent mould on the labels, it is necessary to place a moisture absorber or bag of silica gel in the fridge. 

Fridge Thermostat Circuit diagram :

Fridge Thermostat Circuit Diagram

The circuit is built around an old work-horse among opamps, the 741. D1 pro-vides a stable reference voltage of 5 V across the entire resistor divider. P1 allows adjustment of the voltage at the node of R1 and R2. To use the above-mentioned temperatures as setpoints this voltage needs to be adjusted to 2.89 V. D2 is a precision temperature sensor, which can be used from –40 to +100 °C. The voltage across this diode varies by 10 mV per Kelvin. In this way D2 keeps an eye on the temperature in the fridge. The reference voltage derived from the voltage divider (selected with S1) is com-pared by IC1 with the voltage across the temperature sensor. Based on this, the 741 switches, via the zero voltage crossing driver (IC2), a triac that provides volt-age to the compressor motor. The zero voltage crossing IC switches only at the zero crossings of the mains voltage, so that interference from the compressor motor is avoided when turning on. 

The power supply for the circuit is pro-vided by a simple bridge rectifier and filtered with two electrolytic capacitors of 220 µF each. 

The design can also be used for countless other uses. You can, for example, make a thermostat for heating by swapping the inputs of the opamp.

Keep in mind the safety requirements when building and mounting the circuit.

Detail: Fridge Thermostat

0 Cheapest Ever Motion Sensor

The RS-455-3671 sensor used in the Automatic Rear Bicycle Light project published in  the July/August 2010 edition can be replaced by a motion sensor that costs nothing instead of a fiver or thereabouts. 

 

The replacement is a homemade device, built from components easily found in the workshop of any electronics enthusiast. Effectively it works as a variable resistor, depending on the acceleration force to which the device is  submitted. A prototype presented a resistance of 200 kΩ when not moving, and 190 kΩ when dropping about 1 cm.

 Cheapest Ever Motion Sensor Circuit Diagram

Constructing is easy. Cut off a piece of about 10 mm of copper tubing. Take a piece of conductive foam, the kind used to protect integrated circuits. Cut a rectangular piece of 10  x 50 mm. Roll up firmly until it can be push-fitted securely into the copper cylinder. Then insert a conductive wire through the centre of the cylinder, bend it and (optionally) add protective plastic sleeving to each side. This is the first contact. Finally, solder a thin wire to the copper cylinder. This is the second contact. The foam resistance is pressure dependent. 

Consequently, when the device moves due to an external force, the inertia of the cylinder causes varying pressure in the foam, resulting in a small change of resistance between the inner conductor and  the cylinder. Because of that, it’s important to ensure the cylinder vibration is not restricted in any way by the connecting wire or the PCB. 

The comparator circuit shown here is capable of resolving the resistance change of the proposed foam/wire/copper sensor, allowing it to detect the motion of a vehicle for alarm or other purposes. 

Author : Antoni Gendrau – Copyright : Elektor

Detail: Cheapest Ever Motion Sensor

0 Simple DC Fan Controller

This circuit is ideal to control the cooling fan of heat generated electronic gadgets like power amplifiers. The circuit switches on a fan if it senses a temperature above the set level. The fan automatically turns off when the temperature returns to normal.

The circuit uses an NTC (Negative Temperature Coefficient) Thermister to sense heat. NTC Thermister reduces its resistance when the temperature in its vicinity increases.IC1 is used as a voltage comparator with two potential dividers in its inputs. Resistor R1 and VR1 forms one potential divider connected to the non inverting input of IC1 and another potential divider comprising R2 and the 4.7K Thermister supplying a variable voltage to the inverting input of IC1. VR1 is adjusted so as to give slightly lesser voltage at the non inverting input than the inverting input at room temperature.

DC Fan Controller Circuit Diagram

In this state, output of IC1 will be low and the Fan remains off. When the temperature near the Thermister increases, its resistance decreases and conducts. This drops the voltage at pin 2 of IC1 and its output becomes high. T1 then triggers and fan turn on. Red LED indicates that fan is running. Capacitor C1 gives a short lag before T1 turns on to avoid false triggering and to give proper bias to T1.DC fan can be the one used in Computer SMPS.

Keep the Thermistor near the heat sink of the Amplifier PCB and switch on the amplifier for 10 minutes. Then adjust VR1 till the Fan stop running.When the temperature rises, Fan will automatically switch on. 

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Detail: Simple DC Fan Controller

0 Water Level Alert Circuit Diagram

Water Level Alert Circuit Diagram. This circuit will emit an intermittent beep (or will flash a LED) when the water contained into a recipient has reached the desired level. It should be mounted on top of the recipient (e.g. a plastic tank) by means of two crocodile clips, acting also as probes. If a deeper sensing level is needed, the clips can be extended by means of two pieces of stiff wire (see pictures).

Circuit operation:

IC1, a 555 CMos timer chip, is wired as an astable multivibrator whose operating frequency is set by C1, R1 and R2, plus the resistance presented by water across the probes. If the resistance across the probes is zero (i.e. probes shorted), the output frequency will be about 3Hz and the sounder will beep (or the LED will flash) about three times per second. As water usually presents a certain amount of resistance, the actual oscillation frequency will be lower: less than one beep/flash per second. As probes will be increasingly immersed in water, the resistance across them will decrease and the oscillation frequency of IC1 will increase.

This means that a rough aural or visual indication of the level reached by water will be available. If a LED is chosen as the alert, C2, D1 and D2 must be added to the circuit in order to double the output voltage, thus allowing proper LED operation (see the rightmost part of the schematics). Interesting features of this circuit are 1.5V supply and ultra-low current consumption: 40µA in stand-by and 0.5mA in operation. This allows a single AAA alkaline cell to last several years and the saving of the power on/off switch.

Pictures of the project:

Screenshoot - Water Level Alert Circuit Schematic

Water Level Alert Circuit Diagram:

Water Level Alert Circuit Diagram

Parts:

R1 = 1K - 1/4W Resistor
R2 = 100K - 1/4W Resistor (See Notes)
C1 = 2.2uF-50V Electrolytic Capacitor
C2 = 220µF - 25V Electrolytic Capacitor (See Notes)
D1 = 5 or 10mm. Ultra-bright red LED (See Notes)
D2 = 1N5819 - 40V 1A Schottky-barrier Diode (See Notes)
IC = 7555 or TS555CN CMos Timer IC
BZ = Piezo sounder (incorporating 3KHz oscillator)
B1 = 1.5V Battery (AAA or AA cell etc.)
Two small crocodile clips
Two pieces of stiff wire of suitable length
Battery socket, etc.

Notes:

• If a LED alert is needed instead of the beeper, R2 value must be changed to 10K, the Piezo sounder can be omitted and D1, D2 and C2 must be added, as shown in the rightmost part of the schematics.
• A common red LED can be used for D1, but ultra-bright types are preferred.
• Any Schottky-barrier type diode can be used in place of the 1N5819, e.g. the BAT46, rated @ 100V 150mA.
• Wipe the probes regularly to avoid excessive resistance variations due to partial oxidization.

Source: Red Free Circuit Design

Detail: Water Level Alert Circuit Diagram